The invention relates to liquid polyisocyanate mixtures based on diphenylmethane 2,4′-diisocyanate, to a process for their preparation and to their use in single- and two-component polyurethane coating compositions, adhesives and sealants.
Two-component polyurethane coating compositions based on diphenylmethane 4,4′-diisocyanate are known. For instance, EP-A 0 666 276 describes polyisocyanate mixtures which are liquid at temperatures above 5° C. and are based on diphenylmethane 4,4′-diisocyanate with an isocyanate group content of 14.5 to 24% by weight and a content of allophanate groups (calculated as C2HN2O3) of 7.7 to 14.5% by weight, and also a process for their preparation. These polyisocyanate mixtures are notable for relatively good crystallization stability and high reactivity in combination with reactants containing hydroxyl groups. In the introductory part of the description of EP-A 0 666 276, it is mentioned that mixtures composed of diphenylmethane 4,4′-diisocyanate and relatively large amounts of diphenylmethane 2,4′-diisocyanate are liquid, but have poor crystallization stability, and that the properties of coating materials produced therefrom deteriorate significantly owing to the proportion of diphenylmethane 2,4′-diisocyanate.
For some applications, however, the use of polyisocyanates based on diphenylmethane 4,4′-diisocyanate is undesirable owing to the high reactivity of this isocyanate. Instead, the demand is for polyisocyanates which, in corresponding applications, have similar material properties to those obtainable on the basis of diphenylmethane 4,4′-diisocyanate, but have lower reactivity and hence a longer processing time.
Owing to the isocyanate group in the 2 position, diphenylmethane 2,4′-diisocyanate has lower reactivity towards isocyanate-reactive reactants than diphenylmethane 4,4′-diisocyanate, but diphenylmethane 2,4′-diisocyanate is solid at room temperature, and polyisocyanates based on diphenylmethane 2,4′-diisocyanate also have poor crystallization stability and usually also high viscosities.
It was therefore an object of the invention to provide novel polyisocyanate mixtures which, compared to those based on diphenylmethane 4,4′-diisocyanate, have lower reactivity and significantly better crystallization stability, even at low temperatures below 5° C. This object is achieved through provision of the polyisocyanate mixtures which have allophanate groups and which are characterized in detail below.